The present invention relates to hydrogen absorbing alloy electrodes for use as negative electrodes in nickel-hydrogen cells or batteries.
Nickel-hydrogen cells are available which comprise a negative electrode prepared from a hydrogen absorbing alloy which reversibly absorbs or desorbs hydrogen.
Nickel-hydrogen cells are known as cells having a high capacity, giving a high output and also high in energy density per unit volume and per unit weight.
With the nickel-hydrogen cell, the oxygen gas evolved from the positive electrode during overcharging is consumed by the hydrogen absorbing alloy of the negative electrode. However, when the negative electrode becomes unable to fully consume the oxygen gas owing, for example, to the deterioration of the hydrogen absorbing alloy, the oxygen gas produced is like to increase the internal pressure of the cell or oxidize the hydrogen absorbing alloy.
Accordingly, JP-A No. 287946/1996 discloses an alkali cell comprising a hydrogen absorbing alloy electrode having zirconium oxide added thereto. The zirconium oxide added to the negative electrode improves the gas absorbing performance during overcharging to inhibit the oxidation of the negative electrode. With the oxidation of the negative electrode inhibited, the cell is given increased charge-discharge cycles.
An object of the present invention is to provide a hydrogen absorbing alloy electrode which exhibits a high oxygen gas absorbing capacity during overcharging and which is further improved in charge-discharge cycle characteristics and high-rate discharge characteristics.
To fulfill the above object, the present invention provides a hydrogen absorbing alloy electrode which is prepared from a powder obtained by mixing a hydrogen absorbing alloy powder with a power of at least one complex oxide (It may be called as a double oxide.) selected from the following group.
The group of complex oxides consists of a ZrO2xe2x80x94Y2O3 solid solution, ZrO2xe2x80x94CaO solid solution, CeO2xe2x80x94Gd2O3 solid solution, CeO2xe2x80x94La2O3 solid solution, ThO2xe2x80x94Y2O3 solid solution, Bi2O3xe2x80x94Y2O3 solid solution, Bi2O3xe2x80x94Gd2O3 solid solution, Bi2O3xe2x80x94Nb2O3 solid solution and Bi2O3xe2x80x94WO3 solid solution.
The complex oxides mentioned above are all oxide ion conductors and have many oxygen defects in its crystal structure.
Accordingly, an excess of oxygen gas evolved by the positive electrode during overcharging moves through the electrode and comes into contact with the complex oxide of the negative electrode. Upon contact with the complex oxide, the oxygen gas is ionized and brought into the oxygen defects in the complex oxide, thus penetrating into the surface portion or the interior of the complex oxide.
Since the excess of oxygen gas produced during overcharging is absorbed or adsorbed by the complex oxide of the hydrogen absorbing alloy electrode in this way, this phenomenon prevents the rise in the internal pressure of the cell and inhibits the oxidation of the hydrogen absorbing alloy powder in the alloy electrode during overcharging, affording improved charge-discharge cycle characteristics.
It is thought that hydrogen atoms appearing on the surface of the hydrogen absorbing alloy powder are rapidly oxidized with oxygen atoms released from the complex oxide, consequently accelerating the electrode reaction during high-rate discharge to give improved high-rate discharge characteristics.
Although the hydrogen absorbing alloy powder is not limited specifically in composition, examples of hydrogen absorbing alloy materials suitable for use in the present invention are a hydrogen absorbing alloy material having a crystal structure of the CaCu5 type and represented by MmNixCoyMz (wherein Mm is a misch metal, M is at least one element selected from the group consisting of Al, Mg, Mn, Fe, Sn, Si, W, Zn, Cr and Cu, 2.8xe2x89xa6xxe2x89xa64.4, 0xe2x89xa6yxe2x89xa60.6, 0xe2x89xa6zxe2x89xa61.5 and 4.5xe2x89xa6x+y+zxe2x89xa65.6), and a hydrogen absorbing alloy material having an alloy layer substantially belonging to the Laves phase of an intermetallic compound and a C15-type crystal structure of a cubic system and represented by AB2 (wherein A is at least one element selected from the group consisting of Ti, Zr, Hf, Y, Ca, Mg, La, Ce, Pr, Nd, Nb and Mo, and B is at least one element selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, Zn and Al).
Mm in the hydrogen absorbing alloy mentioned above is important to make the alloy less costly and improved in durability.
Ni forms a crystal structure of the CaCu5 type with Mm and is important because this metal functions to form an alloy crystal lattice serving to absorb and desorb hydrogen in conditions under which the cell is used.
Co affords improved durability during charge-discharge cycles, Al provides improved durability in alkali electrolytes, and Mn acts to increase the amount of hydrogen to be absorbed and desorbed. These elements are therefore important.
It is desired that the hydrogen absorbing alloy powder be at least 1 xcexcm to not greater than 100 xcexcm in mean particle size.
The complex oxide powder comprises an oxygen ion conductor having many oxygen defects in structure. This type of complex oxide material can be at least one of a ZrO2xe2x80x94Y2O3 solid solution, ZrO2xe2x80x94CaO solid solution, CeO2xe2x80x94Gd2O3 solid solution, CeO2xe2x80x94La2O3 solid solution, ThO2xe2x80x94Y2O3 solid solution, Bi2O3xe2x80x94Y2O3 solid solution, Bi2O3xe2x80x94Gd2O3 solid solution, Bi2O3xe2x80x94Nb2O3 solid solution and Bi2O3xe2x80x94WO3 solid solution.
The complex oxide can be produced by preparing a solid solution from two kinds of metallic oxides, for example, by the solid phase process. It is desirable to mix these metallic oxides together in such a weight ratio (mole ratio) that a solid phase can be formed as shown in Table 1 in order to cause many oxygen defects to be present in the resulting complex oxide, whereas even if the metallic oxides are mixed together in a weight ratio exceeding the limit of solid solution, a solid phase is present in the resulting complex oxide to produce an enhanced gas absorbing effect and an increased effect to promote the electrode reaction.
It is desired to mix 0.1 to 10 wt. % of the complex oxide powder with the hydrogen absorbing alloy powder based on the combined amount of the two powders. Use of less than 0.1 wt. % of the complex oxide fails to fully obtain the gas absorbing effect, and the electrode reaction promoting effect during high-rate discharge. Presence of more than 10 wt. % of the complex oxide reduces the absolute amount of the hydrogen absorbing alloy in the electrode to result in a seriously impaired discharge capacity.
Preferably, the complex oxide powder is at least 0.1 xcexcm to not greater than 10 xcexcm in mean particle size.
The hydrogen absorbing alloy electrode can be produced by mixing the hydrogen absorbing alloy powder and the complex oxide powder in respective specified amounts as weighed out, along with a thickener, such as an aqueous solution of polyethylene oxide, and coating a current collector of Ni net or punched metal sheet with the mixture.
A spiral electrode unit can be prepared by winding the hydrogen absorbing alloy electrode into a roll together with a known sintered Ni electrode and an alkali-resistant separator of nonwoven fabric. A nickel-hydrogen cell can be fabricated by inserting the electrode unit into a cell can and placing an aqueous alkali solution obtained by dissolving lithium hydroxide solution in an aqueous solution of potassium hydroxide into the can.
The hydrogen absorbing alloy electrode may have incorporated therein a powder of electrically conductive agent, such as Ni, to give improved conductivity in addition to the hydrogen absorbing alloy powder and the complex oxide powder.
PTFE or like binder may be used in place of the thickener.